In situ neutralization of subsurface acidic pore water

ABSTRACT

A method for neutralizing in situ pore water associated with a subsurface waste material body, the pore water having a pH of less than 2, the method includes the steps of adding a non-carbonate alkali to the pore water to produce a pH-modified acidic pore water having a pH between about 4 and about 5; and adding a carbonate alkali to the pH-modified pore water in an amount sufficient to raise the pore water pH to between about 6 and about 8.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not applicable.

BACKGROUND OF THE INVENTION

[0003] Heavy metal-contaminated waste materials, such as those found indisposed industrial wastes, contaminated soil, and the like, are oftencontaminated with strong acids (e.g., H₂SO₄) having pH values ofapproximately 1.0 or less in associated pore water. This subsurfaceacidic pore water can be neutralized in situ by direct injection of analkali. Neutralization controls mobility of heavy metals in the wastematerials via ground water flow through the waste.

[0004] Typical in situ neutralization methods involve injecting an acidneutralizing agent comprising carbonates, such as Na₂CO₃, NaHCO₃, CaCO₃,MgCO₃, and the like, in the form of an alkaline solution or slurry. Suchprocesses liberate large quantities of CO₂, thereby increasing down holeback pressure and requiring higher injection pressures. The liberatedCO₂ finds preferential pathways to release a pent up excess subsurfacepressure causing “daylighting” of the injected material.

[0005] To avoid daylighting issues associated with carbonate alkaliinjection, non-carbonate alkaline agents such as NaOH, Ca(OH)₂, and thelike can be injected to neutralize the acidic pore water. However,injection of substantial quantities of strong alkaline agents can beproblematic in that the pore water turns rapidly from strongly acidic tostrongly alkaline. This is particularly undesired because a high pHincreases solubility of emphoteric heavy metals and therefore increasesthe potential mobility of such metals. Alternative approaches toneutralizing subsurface acidic pore water in situ are desired.

BRIEF SUMMARY OF THE INVENTION

[0006] The present invention is summarized in that a process forneutralizing highly acidic subsurface pore water in situ includes thesteps of adjusting the pH of the acidic pore water into a range fromabout 4 to about 5 with a non-carbonate alkali agent to produce apartially neutralized subsurface acidic pore water which is thencontacted with limestone (CaCO₃) to bring the pH up to a generallyneutral range of about 6 to about 8.

[0007] The present invention is further summarized in that the methodsof the invention can be practiced on any acidic groundwater orwastewater contained in a tank or pressure vessel where it is desirableto neutralize the groundwater or wastewater while avoiding spillage.

[0008] It is an advantage of the invention that use of a non-carbonatealkali agent eliminates vigorous CO₂ gas generation associated withCaCO₃ neutralization in prior processes.

[0009] It is another advantage of the present invention that the CaCO₃of pore water above a pH of about 4 generates CO₂ at a sufficiently slowrate that the generated CO₂ is dissipated by physico-chemical processes,such as absorption and diffusion that the pore water does not developsubstantial back pressure in the subsurface waste body and the potentialfor day lighting is reduced or eliminated.

[0010] It is a feature of the present invention that the non-carbonatealkaline agents are not used for the entire neutralization process sothat unintended and undesired overshooting of the target pH does notoccur.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0011]FIG. 1 depicts a pH titration of an acidic groundwater, from whichone can calculate a suitable amount of 1N NaOH to employ in asimultaneous addition of non-carbonate and carbonate alkali agents.

DETAILED DESCRIPTION OF THE INVENTION

[0012] The ratio and dosage of the non-carbonate alkali and the CaCO₃can be determined accordingly to the present invention by measuring theinitial pH and acidity of the pore water and then titrating the porewater with caustic solution and CaCO₃.

[0013] CaCO₃ is a preferred carbonate alkali useful in the finalneutralization process because it reacts slowly with the partiallyneutralized acidic pore water and adds a long-term reserve of acidneutralization capacity (ANC) to the soil or waste material bodyassociated with the pore water. CaCO₃ can be obtained from high calciumlimestone, dolomitic limestone, water softening sludge, and otherconventional sources. Other suitable carbonate alkali compounds for usein the final neutralization step include sodium and potassium carbonatesand bicarbonates as well as MgCO₃.

[0014] Advantageously, since preferred non-carbonate alkali agents arehighly reactive while the carbonate alkali agents are slowly reactive inthe pore water, it is possible to add both agents simultaneously. Uponadding a suitable amount to bring the pH of the porewater to the desiredlevel, the non-carbonate alkali agent will react quickly until expendedwithout overshooting the target pH. Thereafter, the slow-actingcarbonate alkali agent will continue to adjust the pH as noted, whileproviding long term pH buffering capacity. For this reason, thecarbonate alkali agent can be added in large excess, whether or not theaddition is simultaneous. More importantly, the amount of non-carbonatealkali should be determined to avoid overshooting the target pH. FIG. 1depicts a pH titration of an acid groundwater, from which one cancalculate a suitable amount of 1N NaOH to employ in a simultaneousaddition of non-carbonate and carbonate alkali agents.

EXAMPLE

[0015] An acidic pore water having a pH of 1.2 and an acidity of 96,000milligrams CaCO₃ per liter was neutralized with limestone alone or witha mix of 1 N sodium hydroxide and limestone with the following results:VOLUME OF PORE WATER pH AFTER (mL) ADDITIVES 2 HOURS OBSERVATIONS 50 2grams high 2.1 Severe foaming calcium limestone 50 25 mL in NaOH + 4.5Very little foaming 2 grams high calcium limestone

We claim:
 1. A method for neutralizing acid water selected from thegroup consisting of porewater, wastewater or groundwater, the waterhaving a pH of less than 2, the method comprising the steps of: adding anon-carbonate alkali to the water to produce a pH-modified acidic waterhaving a pH between about 4 and about 5; and adding a carbonate alkalito the pH-modified water in an amount sufficient to raise the pore waterpH to between about 6 and about
 8. 2. A method as claimed in claim 1wherein the non-carbonate alkali and the carbonate alkali are addedsimultaneously.
 3. A method as claimed in claim 1 wherein thenon-carbonate alkali is selected from the group consisting of NaOH andCa(OH)₂.
 4. A method as claimed in claim 1 wherein the carbonate alkaliis selected from the group consisting of Na₂CO₃, NaHCO₃, CaCO₃, MgCO₃and potassium carbonate.
 5. A method as claimed in claim 1 wherein theporewater is in situ pore water associated with a subsurface wastematerial body.